Underground drilling, such as gas, oil, or geothermal drilling, generally involves drilling a bore through a formation deep in the earth. Such bores are formed by connecting a drill bit to long sections of pipe, referred to as a “drill pipe,” so as to form an assembly commonly referred to as a “drill string.” The drill string extends from the surface, to the bottom of the bore.
The drill bit is rotated so that the drill bit advances into the earth, thereby forming the bore. In a drilling technique commonly referred to as rotary drilling, the drill bit is rotated by rotating the drill string at the surface. In other words, the torque required to rotate the drill bit is generated above-ground, and is transferred to the drill bit by way of the drill string.
Alternatively, the drill bit can be rotated by a drilling motor. The drilling motor is usually mounted in the drill string, proximate the drill bit. The drill bit can be rotated by the drilling motor alone, or by rotating the drill string while operating the drilling motor.
One type of drilling motor known as a “mud motor” is powered by drilling mud. Drilling mud is a high pressure fluid that is pumped from the surface, through an internal passage in the drill string, and out through the drill bit. The drilling mud lubricates the drill bit, and flushes cuttings from the path of the drill bit. The drilling mud then flows to the surface through an annular passage formed between the drill string and the surface of the bore.
In a drill string equipped with a mud motor, the drilling mud is routed through the drilling motor. The mud motor is equipped with a rotor that generates a torque in response to the passage of the drilling mud therethrough. The rotor is coupled to the drill bit so that the torque is transferred to the drill bit, causing the drill bit to rotate.
So called “smart” drilling systems include sensors located down hole, in the drill string. The information provided by these sensors permits the drill-string operator to monitor relevant properties of the geological formations through which the drill string penetrates. Based on an analysis of these properties, the drill string operator can decide to guide the drill string in a particular direction. In other words, rather than following a predetermined trajectory, the trajectory of the drill string can be adjusted in response to the properties of the underground formations encountered during the drilling operation. The technique is referred to as “geosteering.”
Various techniques have been developed for performing both straight hole and directional (steered) drilling, without a need to reconfigure the bottom hole assembly of the drill string, i.e., the equipment located at or near the down-hole end of the drill string. For example, so called steerable systems use a drilling motor with a bent housing in the drilling motor. A steerable system can be operated in a sliding mode in which the drill string is not rotated, and the drill bit is rotated exclusively by the drilling motor. The bent housing or subassembly steers the drill bit in the desired direction as the drill string slides through the bore, thereby effectuating directional drilling. Alternatively, the steerable system can be operated in a rotating mode in which the drill string is rotated while the drilling motor is running. This technique results in a substantially straight bore.
Although steerable systems have been used for many years, these types of systems possess disadvantages. For example, when a steerable system is operated in the sliding mode, the rate of penetration of the drill bit can be relatively low, and stick slip, differential sticking, and difficulties with cuttings removal can be prevalent. Operating a steerable system in the rotating mode can result in an oversize and tortuous bore.
So-called rotary steerable tools have been used over the past several years to perform straight-hole and directional drilling. One particular type of rotary steerable system can include pads located on the drill string, proximate the drill bit. The pads can extend and retract with each revolution of the drill string. Contact the between the pads and the surface of the drill hole exerts a lateral force on the string. This force pushes or points the drill bit in the desired direction of drilling. Straight-hole drilling is achieved when the pads remain in their retracted positions.
Rotary steerable tools can form an in-gauge bore while drilling directionally, and do not posses the disadvantages associated with sliding the drill string. The drill bit in a rotary steerable tool, however, is rotated exclusively by torque generated at the surface and transferred to the drill bit by way of the drill string. Thus, the torque available to rotate the drill string can be limited by drag on the drill string, especially in a highly-deviated bore. Moreover, the drill-bit torque can be further limited by the torque requirements of the hydraulic system that extends and retracts the pads during directional drilling.